The present invention generally relates to bandpass helical filters and, more particularly, to a high-Q, RF, helical filter arrangement utilizing microstrip transmission lines formed on a printed circuit board and having provision for temperature compensation as well as changing the selected frequency range of the filter arrangement.
Helical filters of the type having one or more resonators are, of course, known in the art. Typically, these filter arrangements include a plurality of resonators, or helical coils, encircling a cylindrical core of non-conducting material which are mounted on a suitable rigid base plate. The helical coils themselves are connected to ground at one end only. A tuning slug, threadably received in a top location of an included housing, is adjustable in position with respect to the open end of the helical coil so as to effect a change in the tuning thereof. The coils themselves are included in separate formed compartments within the housing which may be intercoupled by way of apertures in the compartment side walls. It is necessary that there be at least two such helical resonators so that appropriate input and output connections may be provided. A typical filter arrangement comprises three such resonators, the center resonator being coupled to the two end resonators by the referenced apertures in the compartment side walls. It has been customary in the past that the input and output connections be made at a "tap" point on the associated helical resonators for purposes of providing the desired impedance match to the circuit to which the filter arrangement is connected.
One known improvement, directed to just the impedance matching concerns of the filter arrangement, is described in U.S. Pat. No. 4,342,969 to Myers et al., which discloses a way of eliminating the commonly used "tap" on the end helical resonators by providing for each resonator a planar microstrip stub which exhibits an inductive impedance at the resonant frequency of the helical resonator. A summary of such an arrangement is given in column 1, lines 47-68 and column 2, lines 1-13. While such an arrangement does indeed provide an improved impedance match, it does nothing to temperature compensate the overall resonator.
Temperature compensation is necessary because the helical coil expands and contracts with respect to the housing. Moreover, the materials used for the housing and tuning screws expand and contract with changes in temperature. This relative movement between the helical coil and its associated tuning slug threadably held by the housing upsets the electrical tuning thereof.
Prior art structures have commonly effected overall temperature compensation by making use of different materials for each of the constituent parts within the entire filter. By utilizing materials specially chosen for each of the insulating coil forms, the housing, and the tuning slugs, the overall temperature coefficient required for the entire filter is achieved. These materials usually are more exotic and, therefore, more expensive.
Furthermore, if a frequency range change is desired for a given filter, the physical parameters such as coil electrical length and the specific means of input/output impedance matching would have to be modified to restore the desired temperature compensation characteristics. This is a serious disadvantage even when the input/output impedance matching is accomplished by means of a microstrip transmission line on a printed circuit board. So, although the need to effect a change in the coil electrical length may be obviated, this need is transferred upon the microstrip transmission line length to accommodate a desired change in frequency range.
Accordingly, there exists a need for a further improvement in the performance of a helical filter such that it is easily temperature compensated and does not require changes in either coil length or microstrip transmission line length to accommodate a desired change in frequency range.